Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Such pre-ignition events can cause rapid engine degradation. Accordingly, strategies have been developed for early detection of pre-ignition based on engine operating conditions.
One example approach is illustrated by Hashizume in U.S. Pat. No. 5,632,247. Therein, pre-ignition and knock for a cylinder is detected by a knock sensor attached to the cylinder block. Specifically, based on an estimation of the knock sensor reading in two different timing windows, each with differing thresholds, pre-ignition is determined and differentiated from knock.
However, the inventors herein have identified potential issues with such an approach. In one example, the sensitivity of the approach may vary depending on the positioning of the sensor. For example, the sensor location for reliably identifying knock in the cylinder may not align with the sensor location for reliably identifying pre-ignition in the cylinder. In another example, the presence of multiple sensors (one knock sensor per cylinder) adds to component costs without necessarily improving the performance of either knock or pre-ignition detection in the cylinder. As such, the reduced accuracy of engine pre-ignition determination and differentiation (from knock) may lead to rapid engine degradation.
Thus in one example, some of the above issues may be addressed by a method of operating an engine including a plurality of knock sensors distributed on an engine block. In one embodiment, the method comprises, during engine operation, dynamically selecting a knock-indicating sensor from among the plurality of knock sensors for identifying knock in the cylinder, and dynamically selecting a pre-ignition-indicating sensor from among the plurality of knock sensors for identifying pre-ignition in the cylinder, the selections based on operating conditions.
In one example, a vehicle engine may include a first and a second knock sensor distributed at different positions along the engine block. During a first condition (such as based on the engine speed-load conditions, cylinder position, and cylinder firing order), knock and pre-ignition may be identified in a first cylinder based on the first knock sensor while knock and pre-ignition is identified in a second cylinder based on the second knock sensor. In comparison, during a second condition, knock and pre-ignition may be identified in the first cylinder based on the second knock sensor while knock and pre-ignition is identified in the second cylinder based on the first knock sensor. In this way, by assigning different knock sensors to each cylinder based on engine operating conditions, the knock and pre-ignition detection sensitivity for each cylinder can be improved with fewer knock sensors.
In another example, during a first condition, cylinder knock (in any given cylinder) may be identified based on the first knock sensor while cylinder pre-ignition is identified based on the second knock sensor. During a second condition, cylinder knock in the given cylinder may be identified based on the second knock sensor while cylinder pre-ignition is identified based on the first knock sensor. During still other conditions, each of knock and pre-ignition in the given cylinder may be identified based on the first or the second sensor.
It will be appreciated that while the above-mentioned examples illustrate the concept using two knock sensors and two cylinders, this is not meant to be limiting. As such, for a given cylinder, knock or pre-ignition may be identified based on one or more of a plurality of knock sensors distributed along the engine block. Therein, the one or more knock-indicating sensors may or may not overlap with the one or more pre-ignition-indicating sensors.
In this way, by improving the sensitivity of cylinder knock and pre-ignition detection, abnormal cylinder combustion events may be identified more accurately. By enhancing the differentiation of cylinder knock events from cylinder pre-ignition events, appropriate mitigating steps can be taken. By improving the accuracy and response time of pre-ignition detection and mitigation, engine degradation due to pre-ignition can be reduced. By improving the accuracy and response time of knock detection, fuel economy benefits may be achieved. Further, by using the same knock sensor to identify both knock and pre-ignition in each cylinder, synergistic benefits may be achieved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.